Daytime stellar imager

ABSTRACT

An automatic celestial navigation system for navigating both night and day by observation of K-band or H-band infrared light from multiple stars. In a first set of preferred embodiments three relatively large aperture telescopes are rigidly mounted on a movable platform such as a ship or airplane with each telescope being directed at a substantially different portion of sky. Embodiments in this first set tend to be relatively large and heavy, such as about one cubic meter and about 60 pounds. In a second set of preferred embodiments one or more smaller aperture telescopes are pivotably mounted on a movable platform such as a ship, airplane or missile so that the telescope or telescopes can be pivoted to point toward specific regions of the sky. Embodiments of this second set are mechanically more complicated than those of the first set, but are much smaller and lighter and are especially useful for guidance of aircraft and missiles. Telescope optics focus (on to a pixel array of a sensor) H-band or K-band light from one or more stars in the field of view of each telescope. Each system also includes an inclinometer, an accurate timing device and a computer processor having access to catalogued infrared star charts. The processor for each system is programmed with special algorithms to use image data from the infrared sensors, inclination information from the inclinometer, time information from the timing device and the catalogued star charts information to determine positions of the platform. Direction information from two stars is needed for locating the platform with respect to the celestial sphere. The computer is also preferably programmed to use this celestial position information to calculate latitude and longitude which may be displayed on a display device such as a monitor or used by a guidance control system. These embodiments are jam proof and insensitive to radio frequency interference. These systems provide efficient alternatives to GPS when GPS is unavailable and can be used for periodic augmentation of inertial navigation systems.

This application claims the benefit of Provisional Application Ser. No.60/619,858, filed Oct. 18, 2004 and is a Continuation In Part of Utilityapplication Ser. No. 11/106,744 filed Apr. 15, 2005.

FIELD OF INVENTION

This invention relates to stellar imaging systems and in particular tosuch systems useful for position location and platform attitudedetermination.

BACKGROUND OF THE INVENTION

Global positioning systems (GPS) are widely used for navigating shipsand aircraft. However, these systems are vulnerable and have othershortcomings. Their space components are subject to hostile attack andthe systems may be jammed. The systems also suffer from reliabilityfailures and these GPS systems do not provide absolute azimuthpositioning needed for attitude determination. Inertial navigationsystems (INS) mitigate GPS deficiencies; however, these inertialnavigation systems are not accurate over long time periods. Errors mayaccumulate at rates of about an arc-seconds per hour to an arc-minutesper hour. Periodic alignment of the inertial navigation systems isrequired using an external reference system such as a GPS system.

For centuries navigators have used the sky for the most fundamental andaccurate inertial system available, in which each star is a benchmark.Cataloged positions and motions of the stars define the celestialreference frame. The problem is stars are hard to see during thedaytime. Efforts have been made to navigate by stars during daytimeusing very sensitive visible light charge couple device (CCD) cameras,but these efforts as far as we know, have been unsuccessful due to thevery limited number of stars that can be seen with this sensor.

A need exists for a backup to GPS systems and an absolute azimuthreference for fast alignment of INS systems.

SUMMARY OF THE INVENTION

The present invention provides an automatic celestial navigation systemfor navigating both night and day by observation of K-band or H-bandinfrared light from multiple stars. In a first set of preferredembodiments three relatively large aperture telescopes are rigidlymounted on a movable platform such as a ship or aircraft with eachtelescope being directed at a substantially different portion of sky.Embodiments in this first set tend to be relatively large and heavy,such as about one cubic meter and about 60 pounds. In a second set ofpreferred embodiments one or more smaller aperture telescopes arepivotably mounted on a movable platform such as a ship, airplane ormissile so that the telescope or telescopes can be pivoted to pointtoward specific regions of the sky. Embodiments of this second set aremechanically more complicated than those of the first set, but are muchsmaller and lighter and are especially useful for guidance of aircraftand missiles. Telescope optics focus (on to a pixel array of a sensor)H-band or K-band light from one or more stars in the field of view ofeach telescope. Each system also includes an inclinometer, an accuratetiming device and a computer processor having access to catalogedinfrared star charts. The processor for each system is programmed withspecial algorithms to use image data from the infrared sensors,inclination information from the inclinometer, time information from thetiming device and the cataloged star charts information to determineposition of the platform. Direction information from two stars is neededfor locating the platform with respect to the celestial sphere. Thecomputer is also preferably programmed to use this celestial positioninformation to calculate latitude and longitude which may be displayedon a display device such as a monitor. These embodiments are jam proofand insensitive to radio frequency interference. Also these embodimentswork in those areas with poor GPS coverage or where GPS is not availableat all. These systems provide efficient alternatives to GPS when GPS isunavailable and can be used for periodic augmentation of inertialnavigation systems. These systems also provide absolute azimuthmeasurements for platform attitude determination. The invention is basedupon Applicants' discovery that, at infrared wavelengths, a large numberof stars (at positions offset by more than about 30 to 80 degrees fromthe sun) “out-shine” the sky background even at mid-day.

Preferred embodiments of the present invention operate autonomouslyduring daytime and nighttime, utilize observations of stars, and incombination with an inertial navigation system, provide a secondarymeans, independent of radios and GPS, for navigation of aircrafts andships. Preferred processor software includes a background subtractionand a special signal to noise enhancement algorithm, star patternrecognition software, software for mapping of star direction, and analgorithm for computation of the lines of positions, celestial fix, andlatitude and longitude. Preferred software also includesinstrument-control code.

The combination of the present invention with an inertial navigationsystem is a synergistic match. The accuracy of the inertial navigationsystem degrades with time from initial alignment, while the celestialfix accuracy is not time dependent. The inertial navigation systems areoblivious to bad weather, whereas a celestial fix is sensitive to cloudconditions. Both the inertial navigation systems and systems of thepresent invention are passive, jam-proof, and do not depend on shore orspace components. If a run of bad weather interferes with star sights,the inertial navigation system serves as a bad-weather “flywheel” thatessentially carries the stellar fix forward until new observations canbe obtained. Thus, a combination of the inertial navigation system andthe present invention will provide an independent alternative to radiosand GPS.

Systems designed by Applicants include very efficient optical sensors,which increase the probability of detecting stars during daytime byseveral orders of magnitude, as compared with a prior art approach basedon CCD cameras operating at visible wavelengths. The latter is due toseveral factors including:

-   -   a) The number of infrared sources exceeds the number of stars in        the visible waveband,    -   b) The daytime sky background is by a factor of 6-18 lower in        the infrared wavebands than in the visible waveband, and    -   c) The full well capacity of selected infrared sensors is more        than one order of magnitude higher than that for comparable        visible sensors.

Additional advantages of this design approach are associated with thefact that atmospheric obscurants including haze and smoke affectinfrared sensors less than sensors operating in the visible waveband,and the effect of daytime turbulence on the infrared sensor is lower.

In each embodiment of the first set of preferred embodiments each ofthree telescopes are mounted on a moving ship and views a 0.5×0.4 degreeregion of the sky for H-band starlight. Located stars, usually thosewith brightness greater than 6.4 H-band magnitude, are then comparedwith star positions from the star catalog within a selected 5×5 degreeregion of the sky. A correlation of the data from at least two of thethree telescopic measurements determines the position of the platform toa precision of 30 meters. The computer uses this position and the speedof the ship or aircraft, direction and attitude (pitch and roll) toestablish the 5×5 degree region for the next correlation. Applicantshave determined that there are an average of about 300 to 400 daytimevisible infrared stars in these 5×5 degree regions of the Milky Wayportion of the sky and an average of about 30 to 40 visible infraredstars in the 5×5 degree regions in other portions of the sky.Embodiments in this first set of embodiments have no moving parts anduse automatic star detection and star pattern recognition algorithms.These preferred embodiments utilize three infrared telescopes imagingsimultaneously three small areas of the sky, about 45 degrees inelevation with each telescope separated from the others by 120 degreesin azimuth. System software detects and identifies stars and calculatesthree crossing lines of position and a latitude/longitude celestial fix.Ship or aircraft positions may be up-dated every 5 to 10 minutes withposition errors of less than 30 meters.

Embodiments of the second set of preferred embodiments utilizetelescopes with aperture diameter to less than 10 cm to provide muchsmaller and lighter systems. For example, with an aperture diameter of 5cm, the dimensions of the system can be reduced to less than 20 cm×15cm×12 cm. Such a system will fit applications of navigating missiles andaircraft. However, reduction of the telescope aperture diameter willreduce the number of received star photons and the signal-to-noise ratio(SNR). This will reduce the star detection limit, or star-limitedmagnitude. In addition, in order to avoid star image blur due toaircraft or missile motion and vibration, camera exposures on the orderof a few milliseconds should be used. If the camera exposure is reducedto a few milliseconds, then once again the SNR is reduced. Applicantsutilize two techniques to compensate for the above SNR losses caused byreduction of the telescope aperture diameter and camera exposure time.The first technique is to increase the star brightness by pointing atelescope to selected bright infrared stars. In preferred embodimentsApplicants use a two-axis precision rotary stage to point one or moretelescopes at selected bright stars. In these preferred embodimentsApplicants limit the size of search windows to be equal, or less, thanthe 15°×15° square area angular distance over which the selectedcommercially available rotary stage maintains a one arc-second absoluteaccuracy or less. This provides an optical scan of up to 30°×30° whenused with a reflecting mirror. The second approach is to utilizeinfrared cameras with reduced readout camera noise. Applicants havedetermined that for short camera exposures, the SNR is limited by thecamera readout noise rather than the sky background noise. In somepreferred embodiments an “all sky” CCD camera views the entire sky sothat on “partly cloudy days the telescope can be quickly pointed to acloudless region of the sky.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A(1) and 1A(2) show a first preferred embodiment of the presentinvention.

FIG. 1B shows a second preferred embodiment of the present invention.

FIG. 1C shows a third preferred embodiment of the present invention.

FIG. 2A is a graph showing transmission through the atmosphere of lightover a range of wavelengths.

FIGS. 2C(1), 2C(2) and 2C(3) show probabilities of detecting stars.

FIG. 3 shows sky scatter at three wavelength ranges.

FIGS. 4A and 4B describe a telescope.

FIGS. 5A and 5B show a second three-telescope telescope design.

FIG. 6 shows probability of detecting stars as a function of searchtime.

FIGS. 7A and 7B show correlation of star image data with a star chart.

FIG. 8 is a block diagram showing elements of a preferred embodiment ofthe present invention.

FIG. 9 describes elements of a preferred algorithm for navigating bystarlight.

FIG. 10 is an example of starlight images.

FIG. 11 is a graph indicating the number of stars in the entire sky atstar magnitudes between 0 and 7.

FIG. 12 shows a plot of Applicants' measurements of star data ascompared to catalog data.

FIG. 13 shows Applicants' measurements of solar background compared topredictions.

FIG. 14 shows SNR data for a 10 cm diameter aperture telescope.

FIGS. 15 and 16 show similar data for 7.5 cm and 5 cm telescopes.

FIG. 17 shows an optical layout for a preferred single telescopeembodiment.

FIG. 18 shows an optical layout for a preferred two telescopeembodiment.

FIG. 19 shows a layout for an “all sky” embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Rigidly MountedEmbodiments (No Moving Parts)

A first preferred embodiment of the present invention is shown in FIGS.1A(1) and 1A(2). This is a stellar imaging system useful for day andnight accurate stellar navigation for ships. The system is a “strapdown” system (i.e., it is mounted or “strapped down” on a platform, inthis case a ship) with no moving parts. Three telescopes, separated by120 degrees in azimuth and directed at 45 degrees in elevation, provideimages of stars on three infrared 256×320 pixel cameras designed foroperation in the infrared waveband at about 1.6 micron wavelength. Theinstantaneous field of view of each camera is 0.4×0.5 degrees, whichprovides a very high probability of imaging stars that are recognized bya computer system programmed with special algorithms, a star catalog,and star pattern recognition software. The camera is a fast frame ratecamera operating at frame rates up to 30 Hz with a full well capacity of5 million electrons, with thermoelectric cooling.

A second preferred embodiment is shown in FIG. 1B. This embodiment issimilar to the ship version but is designed for aircraft day and nightnavigation. In this embodiment a fewer pixel camera is used providingshorter exposure times to prevent blurring due to faster aircraft motion

The third preferred embodiment is shown in FIG. 1C. This embodiment usesa single telescope to provide accurate azimuth reference for at-seainertial navigation system calibration for attitude determination.

Many more details on the features of these embodiments are provided inthe section below that discusses Applicant's research and these specificfeatures and design choices made by Applicants.

Applicants Spectral Investigations and Selection

Almost all celestial navigation at sea level using starlight has in thepast been at night with observations at visible wavelengths. During theday, sunlight scattered in the earth's atmosphere produces backgroundillumination that makes detection of starlight difficult. Also, strongdaytime sky background quickly fills small electron collection “wells”of visible sensors, thus limiting the aperture diameter and/or exposuretime. As a consequence, visible sensors have small signal to noiseratios and poorer overall sensor performance.

Atmospheric transmittance at wavelengths between about 0.2 microns toabout 3.2 microns is shown in FIG. 2A. Applicants have evaluated andcompared the performance of three candidate sensor systems: oneoperating in the red portion of the visible spectrum (I-band, 0.8 micronwavelength, indicated at 2 in FIG. 2A) and two near-infrared spectralbands (H-band, 1.6 microns wavelength, indicated at 4 in FIG. 2A andK-band 2.2 microns wavelength, indicated at 6 in FIG. 2A).

The analysis included several analytic studies:

-   -   1) Examination of star statistics,    -   2) Evaluation of the atmospheric transmittance and daytime sky        background in three spectral wavebands;    -   3) Evaluation of the effects of atmospheric turbulence, and        atmospheric obscurants on three candidate sensor systems;    -   4) Development of a novel star detection algorithm,    -   5) Testing of the developed algorithm using simulated and field        data;    -   6) Evaluation of commercially available electronic and optical        components and    -   7) A trade-off study to select the best design aproach.

Applicants characterized the overall sensor performance by theprobability of detection of a given number of stars within thefield-of-view of each telescope. They found that the probability ofdetecting stars at daytime with infrared sensors is much higher thanthat with a visible sensor. They determined, therefore, the infraredsensors operating at 1.6 microns or 2.2 microns are the best candidatesystems for the hardware prototype.

Star Statistics

Applicants evaluated the star statistics in the visible I-band by usingthe Catalog of Positions for Infrared Stellar Sources and in the H-bandand K-band by using the 2-Micron All Sky Survey catalog. Both of thesecatalogs are well known and are available on the Internet. They foundthat the number of stars in the infrared wavebands at similar intensitylevels is an order of magnitude greater than the number of stars in thevisible waveband. As an example, FIGS. 2C(1), (2) and (3) show theprobability of detecting at least 1, 2 or 3 stars within a field of viewof 1 degree versus star magnitude in the three spectral wavebands. Forall star magnitudes the probability of detecting stars in the infraredwavebands is about an order of magnitude higher than in the visiblewaveband. This defines the first principal advantage of the infraredsensor.

Atmospheric Transmission and Sky Background

Using a MODTRAN3 computer model available from M. E. Thomas & L. D.Duncan, which is described in “Atmospheric Transmission”, in AtmosphericPropagation of Radiation, F. G. Smith, ed, Vol-2 of The Infrared andElectro-Optical Systems Handbook, J. S. Accetta & D. L. Shumaker, eds,ERIM, Ann Arbor, Mich., and SPIE Press, Bellingham, Wash. (1993), andMODTRAN & FASCOD references cited therein,

Applicants evaluated the atmospheric transmission and daylight skybackground in three candidate spectral wavebands (1.6 micron-0.25 micronbandwidth, 2.2 micron-0.2 micron bandwidth and 0.8 micron-0.1 micronbandwidth). The total atmospheric transmission in the infrared wavebandsis 20 to 30 percent higher than the visible. Sky background radiationbased on Applicants' calculations is shown in FIG. 3. The sky backgroundradiation at potential wavelength ranges is plotted versus angulardistance between the sun and the detector pointing direction for twoatmospheric conditions (23 kilometers visibility and 10 kilometersvisibility). They found that the daylight sky radiance in the infraredwavebands is significantly lower than that in the visible waveband. Thesky radiance in the H-band and K-band is lower by a factor of 6 and 18,respectively, than that in the I-band. In addition, the averageatmospheric transmittance in the infrared wavebands is higher andeffects of atmospheric obscurants including haze, smoke, and clouds thatcan attenuate starlight is also lower in the infrared waveband than thatin the visible. Thus, in the IR waveband there is less atmosphericscattering and higher transmission. This provides the second principaladvantage of the infrared sensor. Total sea-level transmission throughthe atmosphere as a function of wavelength is shown in FIG. 2A.

Daytime Sea Level Turbulence

The effect of daytime sea-level turbulence on the infrared sensor islower than that in the visible waveband. In particular,turbulence-induced scintillation at daytime can cause strong signalfades at the detector and thus degrade the performance of the visiblesensor. The scintillation index, or normalized log amplitude variance,that characterizes the effect of turbulence on the star image brightnessis reduced by a factor of 2.2 and 3.2 in H-band and K-band, as comparedto I-band. Consequently, the effect of scintillation on an infraredsensor is expected to be small. Also the atmospheric coherence diameter,or Fried parameter, that characterizes turbulence-induced image blur, isincreased by a factor of 2.3 and 3.4 in H-band and K-band, as comparedto I-band. This defines the third advantage of the infrared sensor.

Full Well Camera Capacity

The fourth principal advantage of the infrared sensors is associatedwith the fact that the infrared cameras typically have a large full wellcapacity. The full well capacity of the infrared sensors exceeds thevalue for the CCD visible sensors by more than one order of magnitude (5to 20 million electrons in the infrared waveband vs 0.1 millionelectrons in the I-band). A large full well capacity is extremelyimportant for daytime operations. It allows Applicants to increase thesignal to noise ratio for the infrared sensors by increasing theaperture diameter (up to 20 cm) and/or integration time to successfullydetect stars in the presence of a strong sky background. Conversely, thesmall full well capacity of the visible sensor limits the aperturediameter and the total exposure and thus limits the signal to noiseratio, star detection limit, and probability of detecting stars. Largefull well capacity is the fourth advantage of the infrared sensor.

Camera Frame Rate

The infrared sensors have much higher full frame rate, than visiblesensors. Due to large pixel count (4096×4096 pixels) required to image alarge field of view, the frame readout period is 3.5 sec in the visible,while it is typically 30 msec in the infrared waveband. This allowsApplicants to increase the signal to noise ratio by averaging multipleframes. The accuracy of the image centroid calculations is determined bythe image spot diameter D_(star) and the signal to noise ratio.$\sigma = {\frac{3\pi}{16}\frac{D_{star}}{SNR}}$

The signal to noise ratio is given by${{SNR} = \frac{N_{S}}{\sqrt{N_{S} + {4\left( {n_{B} + n_{D} + n_{e}^{2}} \right)}}}},$where N_(s) is the total number of signal photoelectrons detected in aframe (assuming within an area of 4 pixels and that the spot size fullwidth at half maximum is approximately 1 pixel), n_(B) is the number ofsky background photoelectrons detected per pixel, n_(D) is the number ofdark current electrons per pixel, and n_(e) is the number of read noiseelectrons per pixel.

Averaging of multiple data frames by using a shift-and-add techniqueprovided an additional way to increase the signal to noise ratio. Thesignal to noise increases proportionally to √{square root over (N)},where N is the number of averaged frames. An implementation of thistechnique with the infrared sensors is straightforward because the framerate for the IR cameras is approximately 5 to 30 Hz, depending on theexposure time. This provides the fifth advantage of operating in theinfrared spectrum.

H-Band and K-Band are the Spectral Ranges of Choice

The above performance analysis revealed that the infrared sensor, ascompared to the visible sensors, have a much higher probability ofdetecting stars. In particular, in a clear atmosphere for optimalaperture diameter and optimal angular pixel size the star detectionlimit for the I-band sensor is magnitude 3.3, whereas for the H-band andK-band sensors it is 6.8 and 5.8, respectively. The optimum field ofview of the I-band sensor is 7×7 degrees, whereas the optimum field ofview of the H-band and K-band sensors is 0.86×0.86 degrees and 1.3×1.3degrees. For given sensor parameters, Applicants found that theprobability of detecting at least 1 star with a 4096×4096 pixel I-bandsensor is 0.18, whereas the probability of detecting at least 2 stars is0.03. Under the same conditions, using a 512×512 pixel H-band sensor,the probability of detecting at least 1 star is 0.86 and probability ofdetecting at least 2 stars is 0.62. The number of infrared sources(H-band or K-band) of magnitude 7 is about 350, 000 in the entire sky,whereas the number of I-band stars of magnitude 3.3 in the sky is onlyabout 300. Thus, the probability of detecting stars using infraredsensor is higher than using the sensor operating in the visiblewaveband. Therefore, in sense of performance and utility for the surfacefleet and aircraft navigation, the infrared sensors have greater valuethan the visible sensors.

Each of the three telescopes scan a region of the sky and the regiongrows with time. (The sky appears to rotate 1.25 degrees each fiveminutes.) FIG. 6 shows probability of detecting of at least 1, 2 andmore stars in a 1×1 degree field of view with an infrared sensor thathas star detection limit of 6.4 magnitude. The probability of detectingstars is shown versus observation time. The probability of detecting atleast 1 star in the field of view exceeds 90% for the observation timeof 5 minutes. For comparison, a strap-down system operating in thevisible waveband and having field of view of 7×7 degrees and stardetection limit of 3.3 magnitude will require 4-6 hours to detect atleast one star.

In summary, Applicants' trade-off study revealed that the infraredsensor has an inherent advantage, based on the laws of physics, over theprior art visible sensor in probability of detecting stars.

Daylight Stellar Imager Sensor Optimization

Applicants' trade-off studies included a comparison of the H-band andK-band sensors in terms of detector format, cost, and coolingrequirements. Applicants found that the H-band (InGaAs) sensor fromSensors Unlimited, which operates in the 0.9 -1.7 micron spectral band,has sensor performance somewhat (but not much) better than to the K-bandin terms of star detection probabilities, but this H-band sensor hasseveral more important advantages over the K-band sensor. First, it isless expensive ($25K for a 320×256 InGaAs array from Sensors Unlimitedversus $120K for a 256×256 HgCdTe sensor from Rockwell). Second, itrequires only TE cooling to obtain low dark current levels for low noiseperformance and does not use liquid nitrogen. Third, its full wellcapacity of 5 million electrons is greater than the full well capacityof K-band sensors considered. For these reasons, the Sensors UnlimitedMinicamera 320×256 pixels camera was selected for the hardware prototypeof Applicants' first preferred system.

Applicants also determined the optimal number of fields of view to besimultaneously viewed, optimal sensor pixel size, and the field of viewangular size. We found that the optimal pixel size in the H-band isapproximately 6 arc-sec. Regarding the number of fields of view,Applicants considered two options:

-   -   a) using one field of view and taking sequential stellar        measurements at different areas of the sky, or    -   b) using 3 fields of view and three cameras and doing        simultaneous measurements.

Due to the effects of vibration and ship/aircraft motion on theline-of-sight between sequential stellar measurements, Applicantsdetermined that simultaneous measurements with three fields of view arepreferred for a hardware prototype. Each field of view is 0.4×0.5degree.

Single Aperture Daylight Stellar Imager Opto-Mechanical Design

A single aperture telescope was constructed and star observations wereperformed at sea level at daytime. Images of known stars were taken andstored for post-processing to determine photon flux levels in the K-bandor minimum detectable stellar magnitudes. Multiple data sets werecollected for various atmospheric transmittance and angular distancefrom the sun.

FIG. 10 shows one example of the daytime K-band detection of stars withbrightness values ranging from about 6.3 to 1.8 at an angular distanceof 100 degrees from the sun. Seven stars are detected in the field ofview of 0.4×0.5 degrees. These measurements confirm that minimumdetectable stellar magnitude for the K-band sensor at daytime is about6.4 to 6.9.

Multi-Aperture Daylight Stellar Imager Optical-Mechanical Design

A first preferred embodiment is a device which can autonomouslydetermine its geographical position with a horizontal position error ofless than 30 meters both day and night purely from observations of starsand deliver a latitude/longitude fix every 5 minutes. This embodiment isshown in FIGS. 4A and 4B. It is a prototype designed as amulti-aperture, strap down system without moving parts. Themulti-aperture optical-mechanical design is a direct extension of thesingle aperture design. The same 20 cm telescope system and threeinfrared cameras are used. Each of the three apertures are mounted to aHoltzen parallelpiped, providing a line-of-sight that is at the samezenith angle of 45 degrees (from the horizon) with a 120 degrees offsetin azimuth between each of the three apertures. The use of threeindependent apertures allows for both increased positional accuracy dueto the ability to triangulate the measurements and redundancy in caseone of the apertures line of sight is close to the sun. The structuralsupport of the optics can be a simple aluminum or fiberglass tube, butcarbon fiber composites may be desired for better thermal performance.The tube extensions (beyond the first optical element) act as sunbaffles. The performance of the system is maintained so long as directsunlight does not scatter into the telescope. An even longer bafflewould allow operation slightly closer to the sun, but the 30 degreebaffle shown is adequate under most circumstances. FIG. 4B shows theoptical components of one of the three telescopes. As shown in FIG. 4Athe optical axes of three telescopes are intersected to minimize thesystem foot print and total dimensions. The cameras are fastened to thetelescope structure normal to the optical axis as indicated at 10 inFIG. 4B. A triangular frame at the bottom as shown at 12 in FIG. 4Aprovides structure rigidity. The entire assembly is meant to have thesame low expansion coefficient, so if the entire structure is shadedfrom direct sunlight and if the structure remains isothermal, then theangle between the telescopes should remain fixed. The total weight ofthis strap down assembly is about 120 to 140 pounds.

Platform Attitude Determination

Star measurements with a single telescope can provide absolute azimuthreference for platform attitude determination. Once a star in the fieldview is detected and identified, a corresponding line of sight isaccurately known. A projection of this line of sight on the horizontalplane defined as cos(star azimuth angle) provides an absolute azimuthreference. A sensor system with three telescopes provides threeindependent azimuth references that can be averaged together to reducethe measurement noise. Platform attitude determination does not requiremeasurement of a local vertical with an inclinometer.

Analysis and Algorithm Development

To further improve the sensor performance and reduce the star detectionlimit, Applicants developed a robust image processing algorithm. Thisalgorithm allows Applicants to accurately determine star position in theimagery data recorded in the presence of a strong sky background andhaving low contrast and low signal to noise ratio. The basic concept isthe following. The algorithm uses the fact that the pixels which includethe star image are illuminated with spatially correlated light (photonsall coming from the same source, a single star), whereas pixels that areilluminated with only sky background are illuminated with spatiallyuncorrelated light (randomly scattered photons from the sun). Therefore,if the signals in the neighboring pixels are summed up, pixels with thestar image and the noisy pixels will have different gain, and thus canbe distinguished. If N_(p) pixels are summed up, then the signal levelin the super-pixel with the star light will increase proportionally to,N_(p), whereas the signal level in the super-pixel that include noiseincreases proportional to √{square root over (N_(p))}. Thus, the signalto noise ratio increases by a factor of √{square root over (N_(p))}.Similarly, when N image frames are summed up, the signal to noise ratioincreases by a factor of √{square root over (N)}.

The image processing algorithm includes two stages: a) backgroundsubtraction and noise reduction stage and b) star detection and centroiddetermination stage. For Applicants' prototype unit, each data frame istime stamped using a time reference instrument provided by Inter-RangeInstrumentation Group (IRIG) and an off-the-shelf personal computerinterface card. The IRIG system relies on GPS for time determination buthas an AM radio backup in the case GPS is not available. The time isaccurate to within 1 microsecond, which is very small compared topreferred timing requirement of 10 milliseconds (corresponding to 5meters in platform position error). Since it is important for manyapplications that the system be independent of GPS, Applicants willreplace the IRIG time base with an alternate clock source which isindependent of GPS. Atomic clocks are standard equipment on many shipscould provide the alternate clock with sufficient accuracy. Thesealternate timing sources can be reset with GPS or radio when the resetsources become available.

The image processing algorithm includes the following steps:

-   -   1. Median value subtraction. This step reduces the fixed pattern        camera noise. Consider the data set that consists of 20 frames.        First, using 20 data frames that precede the first frame in the        data set, the median data is calculated to for each pixel. Then,        the median frame of pixels is subtracted from each frame in the        data set, pixel by pixel.    -   2. Next, to reduce noise, five sequential data frames in the        data set are blindly summed up. This typically spreads star        illumination over a few pixels.    -   3. Then a “super pixeled” image is created by down sampling the        image generated in step 2 at the rate of 1:4 (i.e., four        adjacent pixels are summed across the pixel array)    -   4. Determine the brightest super pixel in the first frame from        step 1 and create a small (9×9 regular pixel size) window about        the brightest super-pixel location (81 pixels with the brightest        4×4 in the middle).    -   5. To increase the centroid accuracy, up sample the image within        the window at the rate 10:1 using cubic spline fit algorithm.        (The computer produces a digital array of 90×90 [8100] virtual        pixels and fits them with the cubic spline fit algorithm into a        Gaussian-like shape.)    -   6. Calculate the intensity weighted centroid. Under this step an        expected star location in the first data frame is determined.    -   7. Repeat steps 5-6 for each subsequent data frame in the data        set.    -   8. Once an expected star location in all subsequent data frames        are determined, shift all 20 frames to the star position in the        first frame, and sum up all frames. This step produces the final        image for star detection within the 9×9 pixels window. The        extent of the shift is based on the location of the centroid.    -   9. Once an expected star location in all subsequent data frames        are determined, shift all 20 frames to the star position in the        first frame, and sum up all frames, The final image for the        entire frame is based on the shifts obtained from the 9×9 pixel        centroiding window. The extent of the shift is based on the        location of the centroid.    -   10. Create a “super pixel” representation of the shifted and        added frame obtained from step 8 by down sampling at the rate of        1:4 (i.e., four adjacent pixels are summed across the pixel        array). Determine brightest super pixel and create a small (9×9        regular pixel size) window about that location.    -   11. Up sample the image within the small 9×9 pixel window with a        ratio of 10:1 by using cubic spline fit algorithm. Remove        background by chopping at noise ceiling, calculate intensity        weighted centroid position as well as total intensity in the        image. Make an estimation of the rms noise by taking the        standard deviation σ of all pixels [other than pixels        illuminated by bright stellar objects] in the entire image        frame. Remove the data within the small window in order to        search for the next dimmest star. Repeat steps 9 and 10 until        all potential stellar objects within the frame are found.    -   12. For each potential star location, the pixel SNR is        calculated:        ${{SNR} = \frac{I_{S} - \left\langle I \right\rangle}{\sigma}},$        where I_(s) is the total signal intensity divided by number of        pixels in the image, <I> is the mean intensity in the image, and        σ is the rms noise. If the SNR≧10, then the star is detected.        The star coordinates are determined by intensity weighted        centroid calculated in step 11. If SNR<10, then this potential        star location is rejected and treated as a noise.    -   13. The star coordinates alone with the star intensity        calculated in step 11 are used further by automated star pattern        recognition algorithm. Also the coordinates of the brightest        star in the field of view are used in calculations of the        latitude/longitude celestial fix and absolute azimuth        determination.

The above algorithm was tested on both simulated data and field data.Applicants found that the algorithm allows us to detect 6.4 magnitudestars in the imagery data recorded at sea level at daytime. They alsofound that the measured distances between stars agree with their catalogvalues to the accuracy of 0.5 arc-seconds.

Star Catalog Development

Stellar identification and celestial latitude/longitude fix calculationsrequire the infrared star catalog that includes accurate star positions,motions, and magnitudes (apparent brightness). Researchers from US NavalObservatory based on the 2MASS catalog and other sources availableprovided the IR star catalog that includes about 350,000 stars down to7^(th) magnitude. The H band magnitude corresponds to the 1.6 μmwaveband where the camera is sensitive. Only objects brighter than orequal to the 7^(th) magnitude were included in order to limit the diskspace required to store the data.

Automated Star Pattern Recognition Algorithm

Using star positions and star relative brightness alone with thetriangle patterns, the stars in each field-of-view are identified usingreference catalog of positions and relative brightnesses, which is asubset of the infrared catalog. The reference star catalog currentlycovers the entire sky with 350,000 stars visible in the infrared. Thefield of view of each of the three telescopes is an area of the sky of5×5 degrees centered about the pointing direction for each telescopedetermined based on the inclinometer measurements of the local horizon,and the angular separation of the three fields. When looking at theMilky Way the number of stars in the 5×5 field is about 300 to 400 andin regions of the sky other than the Milky Way the number of stars isabout 30 to 40. In another embodiment the fields of view are increasedto 10×10 degrees. All star catalog positions are corrected to thecurrent epoch and corrected for proper motion. The distances between allstar pairs in the reference catalog are calculated.

After that the measured distances between all star pairs detected in thefield of view are calculated. The stars detected within the field ofview are listed in descending order, where the brightest stars arelisted first. The first star pair would represent the brightest twostars. Position of each star is corrected for atmospheric effects andstellar aberration. Then the distances between all star pairs arecalculated. Next the measured distances between stars are compared withthe distances from the reference catalog. In order to accommodate thecentroid measurement errors and effects of turbulence of a star image, a5 arc-seconds error is allowed.

In addition to the distances, each observed pair of stars also include aratio of the relative intensities. The measurements performed by theApplicants revealed that individual star measurements fit the curve$\begin{matrix}{{M_{2} - M_{1}} = {0.4*{\log_{10}\left( \frac{I_{1}}{I_{2}} \right)}}} & (1)\end{matrix}$with an error of 0.5 star magnitude. Here M₁ and M₂ are the starmagnitudes from infrared catalog, and I₁ and I₂ are the measured starintensities.

By using these two criteria, only the star pair, which matches thecatalog distance within the accuracy of 5 arc-sec, and also theirmeasured relative intensities match the ratio of the catalog intensitieswithin the error of 0.5 star magnitude are accepted. If there are morethan two stars in the field of view, then once the pair 1-2 is correctlyidentified, the search for each subsequent star's distance as related tostar one and two, i.e. 1-3, 2-3, 1-4, 2-4, etc, is performed. The majorchange in the identification of these stars is the use of an additionalconditional statement that includes a triangle pattern.

Each subsequent pair must include either star one or star two, otherwisethis star is rejected. This creates a form of a triangle pattern, wherestars one and two present two of the three points. The third point ineach triangle is the next star in question. This algorithm wassuccessfully tested on the field data recorded at both day and night.FIGS. 7A and 7B show one example of the stars identified from the fielddata recorded at daytime (FIG. 7B) and compared with the star map (FIG.7A) from the infrared catalog. Six stars having brightnesses varyingfrom 3.4 to 6.6 magnitude are detected and identified. The 7^(th) starin the field of view that has a brightness of 7^(th) magnitude was noteasily detected. Finally, if a single star is detected in the field ofview, then the algorithm will use the relative magnitudes and positionsof stars in all three fields for star identification.

Sensor Software and Electronics

A simple block diagram of the electronics is shown in FIG. 8. All of thecomponents are controlled by software written on a standard personalcomputer 40. The interface to the camera 41 is achieved using a framegrabber board (not shown) on the personnal computer interface bus withoff-the-shelf software drivers provided by Sensors Unlimited. Each frameis time stamped. A commercial inclinometer 46, currently base lined as aunit from Jewel Instruments, is used to provide the local horizonmeasurement necessary to determine the elevation angle of the detectedstars. The inclinometer provides a pair of analog voltages proportionalto the tilt in each of two axes. The tilt meter output is digitized byan off the shelf analog to digital converter 48 synchronized to thecamera frame acquisition. The analog to digital converter is also usedto digitize the output of an off-the-shelf Meteorological Stationsystem. The temperature and pressure are preferably used to correct thestellar position measurements for atmospheric refraction. For elevationangles (greater than 10 degrees), the atmospheric refraction is afunction only of the local index of refraction which can be predictedaccurately knowing only the wavelength of light, and the temperature andpressure.

All software runs on standard personal computer 40. As a baseline thesoftware is written in C++. A flow chart of the software to operate thecamera, to process the frames, and to determine the longitude/latitudecelestial fix is shown in FIG. 9. A single exposure from the camera istransferred from the frame grabber board to the personal computer usingsoftware drivers, and is time stamped from the IRIG time base. Using theimage processing algorithm described above, the stars in each field ofview are detected. The stellar positions within each field are thencorrected for atmospheric refraction. Then using the stellar positionsand relative brightness along with the triangular patterns the stars areidentified.

After that longitude and latitude celestial fix is determined using themeasured stars elevations from at least two of the three fields.Applicants use all three when they are available. When several stars aredetected within the field of view, the elevation of the brightest staris used in position fix calculations. The fix calculations are performedusing the engine from the STELLA software developed at the US NavalObservatory. (J. A. Bangert, “Set Your Sights on STELLA: New CelestialNavigation Software from US Naval Observatory, Chips, Vol. 14, No. 5, pp5-7 (1996). This software calculates both celestial positions andlatitude and longitude for the platform, as well as the platform speedand direction.

The Use of the Stellar Measurements

The obtained celestial position fix provides a back up for GPS, when theGPS is not available. In addition, it will provide periodic alignmentsfor the inertial navigation system to correct for the drifts andlatitude bias. In preferred applications the present invention isintegrated with the inertial navigation system. This helps to mitigatean impact of a cloud cover on the performance of the present invention.If bad weather separates star sights, the inertial navigation systemwill carry the stellar fix forward until new observations can beobtained. Finally, each star measurement provides an absolute azimuthneeded for platform attitude determination.

Kalman Filter

Kalman filtering is a preferred method for estimating, or updating theprevious estimate of a system's state by: (1) using indirectmeasurements of the state variables, and (2) using the covarianceinformation of both the state variables and the indirect measurements.The basic idea is to use information about how measurements of aparticular aspect of a system are correlated to the actual state of thesystem. The Kalman filter estimates a process by using feedback control:the filter estimates the process state at some time and then obtainsfeedback in the form of (noisy) measurements. Accordingly, the equationsfor the Kalman filter fall into two groups: time update equations andmeasurement update equations. The time update equations are responsiblefor projecting forward (in time) the current state and error covarianceestimates to obtain the a priori estimates for the next time step. Themeasurement update equations are responsible for the feedback, i.e., forincorporating a new measurement into the a priori estimate to obtain animproved a posteriori estimate.

Kalman filtering is an important tool in many navigation systems.Indeed, the Kalman filter can be used to integrate the present inventionwith an inertial navigation system (INS). The INS is considered to bethe system model and its outputs are regarded as the referencedtrajectory. Measurement aids, including data from the present invention,are used to compute errors and they are applied to the reference togenerate the combined output. The filter can accept as data theestimates and covariance matrices for vessel coordinates and sourcepositions generated from the analysis of the primary observations.Similarly, it can be used as an observer in a feedback system fordisturbance rejection (and hence smoothing a vessel's motion) usingestimates of the vessel coordinates, since tracking and outputdisturbance attenuation are essentially equivalent problems (at leastfor linear models).

Alternate Telescope Designs

An alternative design approach for the multi-aperture unit uses a singleinfrared camera with large pixel count and is required to combine thelight from each of the three independent apertures on a single detectorarray. The preferred technique uses a small turning mirror and 3-sidedpyramid mirror to combine the light from the different apertures. FIGS.5A and 5B show the design of a pyramid mirror combining system forcombining three celestial beams onto a single infrared sensor 40 locatedat the focal plane of each telescope. The light from each lens assemblyis first reflected off a small turning mirror and then a three-sidedpyramid shaped mirror placed directly in front of the camera array.These pyramid assemblies are typically polished from a solid glasssubstrate and are generally used in the opposite direction as solidretro-reflectors. In this design, the outer glass surfaces will becoated with an enhanced aluminum coating for high reflectivity in theH-band. FIG. 5B also indicates how a larger 640×512 array is separatedinto the three distinct regions for the different apertures with thepyramid mirror. Only two regions 40A and 40B are shown. Each individualaperture uses approximately ⅓ the entire array area with an effectivefield of view of a 0.55 degree square (or 0.62 degree circular).

Another aperture combining technique investigated by Applicants involvesthe use of bent fiber image conduit. This requires the infrared camerato be modified so that the thermoelectric cooler package (that normallyhas a window in front of the array) would be replaced with a fiberwindow bonded directly to the array. Due to this additional expense, thepyramid mirror technique was selected as the preferred aperture combinerfor the alternative preferred embodiment.

Compact Daytime Stellar Imaging Embodiments (with Pointing Telescopes)

Embodiments of the present invention described above tend to berelatively large and heavy, such as about one cubic meter and about 60pounds. A need exists for embodiments of the present invention that aresmall enough to fit easily in an aircraft or missile. Described indetail below are a second set of preferred embodiments utilizing one ormore smaller aperture telescopes that are pivotably mounted on amovable. platform such as a ship, airplane or missile so that thetelescope or telescopes can be pivoted to point toward specific regionsof the sky. Embodiments of this second set are mechanically morecomplicated than those of the first set, but are much smaller andlighter and are especially useful for guidance of aircraft and missiles.In addition, these embodiments have high update rates.

This second set of embodiment like the first set is based uponApplicants' discovery that, at infrared wavelengths, a large number ofstars (at positions offset by more than about 30° from the sun)“out-shine” the sky background at sea level even at mid-day. The midwaveinfrared cameras provide several principal advantages compared to thevisible waveband sensors. This includes:

-   -   The number of bright infrared stars exceeds the number of stars        in the visible waveband by about one order of magnitude    -   There is less atmospheric scatter and higher transmission at        longer wavelengths.

The daytime sky background at 1.6 μm and 2.2 μm is lower by a factor of8 and 16, respectively, compared to the visible waveband

-   -   The effects of atmospheric obscurants and daytime turbulence are        lower at longer wavelengths compared to the visible    -   The full well capacity of infrared cameras is about one order of        magnitude greater than for visible waveband sensors (CCDs). This        allows us to increase the aperture diameter and/or the exposure        time, thus increasing the signal to noise ratio.

However, a large (20 cm) diameter aperture leads to large sensordimensions and large weight because size of the stellar imager, as wellas its weight, depends on the telescope aperture diameter. In order toprovide a compact and light-weight optical sensor, Applicants reduce thetelescope aperture diameter to less than 10 cm. For example, if theaperture diameter is 5 cm, then the sensor dimension can be reduced toless than 20 cm×15 cm×12 cm. Such a sensor will fit applications ofnavigating missiles and aircraft.

However, reduction of the telescope aperture diameter will reduce thenumber of received star photons and the (SNR). This will reduce the stardetection limit, or star-limited magnitude. In addition, in order toavoid star image blur due to aircraft or missile motion and vibration,camera exposures on the order of a few milliseconds should be used. Ifthe camera exposure is reduced to a few milliseconds, then once againthe SNR is reduced. The implication is that a reduction of a telescopeaperture diameter and camera exposure time will reduce the SNR and thuswill limit our ability to detect stars in the presence of strong skybackground.

One can compensate for the above SNR losses caused by reduction of thetelescope aperture diameter and camera exposure time by using twoapproaches. The first approach is to increase the star brightness bypointing a telescope to selected bright IR stars within a search window.The second approach is to reduce the readout camera noise because forshort camera exposures, the SNR is limited by the camera readout noiserather than the sky background noise. Both techniques are discussedbelow.

In order to detect stars at sea level in the presence of strong daytimesky background with a small-aperture telescope and short cameraexposures, Applicants use a strap-down optical system with a limiteddegree of freedom, which includes a two-axis precision rotary stage andallows us to point the FOV of a telescope at selected bright starswithin a search window. Applicants limit the size of a search window tobe equal, or less, than the angular distance over which the selectedrotary stage maintains a one arc-second absolute accuracy or less.Commercially available single axis rotary stages (from Aerotech, Inc.,for example) provide a 1 arc-second absolute accuracy over a mechanicalangle of 15°×15° square area, providing an optical scan of up to 30°×30°when used with a reflecting mirror. By combining two stages with a rightangle bracket and performing a calibration, we can provide biaxialperformance with 1 arc-second accuracy. Applicants use these rotarystages, or similar devices, in their compact stellar trackers. Also,Applicants limit the size of a search window to 15°×15° to minimizepossible calibration errors.

Performance Analysis

Because precision rotary stages have very high absolute accuracy (1arc-second) over a limited optical angle of 15°, the search window islimited to a 15°×15° square area. In order to evaluate the sensorperformance, Applicants determined the average number of bright infraredstars within a 15°×15° area of the sky. FIG. 11 depicts the number ofH-magnitude stars vs. star magnitude over the entire sky. Using the datafrom FIG. 11, one can calculate the average number of stars of a givenmagnitude in the area of the sky of any size.

If N is the number of stars in the sky, and A is the area of the sky insquare degrees that corresponds to the search window, then the averagenumber of stars in this area isn=N×A/41253

since there are 41253 square degrees on the sky. Here A=V², where V isthe width of the search window in degrees. The average number of starsof different H-band magnitude in a 15°15° window is shown in Table 1.TABLE 1 Average Number of Stars of Different H-magnitude in a 15° × 15°window Star Magnitude 1 2 3 4 Average Number of Stars 3 11 30 98

As indicated in Table 1, there are on average 3 to 30 stars of first tothird H-band magnitude in the 15°×150 square window. Since only a fewstars are required to determine a celestial fix, this means that thereare enough bright infrared stars in the 15°×15° search window fornavigating missiles and aircraft using a proposed compact optical GPSunit.

Next, Applicants determined the star detection limit for the proposedstellar tracker, or H-band star magnitude that the proposed stellartracker can detect at daytime at sea level. Applicants performed SNRcalculations using field data acquired with an H-band sensor with anequatorial mount that can be pointed at any direction on the sky. Thenumber of star photoelectrons versus H-band star magnitude measured atsea level at daytime at 80° angular distance from the sun is shown inFIG. 12. The measurements were performed at daytime at sea level atangular distance from the sun of 80° using a 20 cm aperture telescopeand an infrared camera, which is sensitive in the spectral waveband from1400 nm to 1700 nm. The measured signal intensities in FIG. 12 inphotoelectrons/millisecond are compared with the corresponding valuesfrom an infrared catalog. The measured signal intensities agree with acatalog values with an accuracy of 0.5 magnitude.

FIG. 13 compares the measured daytime sky background with thetheoretical prediction using MODTRAN3 code for 23 km visibility. Themeasurements were performed using the same infrared camera and a 20 cmaperture telescope at sea level. The number of measured backgroundphotoelectrons in photoelectrons/pixel/millisecond is shown versusangular distance from the sun. The measurements and theoreticalpredictions are consistent with each other.

By using the measured data shown in FIGS. 12 and 13, SNR calculationswere performed. The signal to noise ratio was calculated using theequation:${{SNR} = \frac{N_{S}}{\sqrt{N_{S} + {4\left( {n_{B} + n_{D} + n_{e}^{2}} \right)}}}},$where N_(S) is the total number of signal photoelectrons detected in aframe (assuming an area of 4 pixels and that the spot size full width athalf maximum is approximately 1 pixel), n_(B) is the number of skybackground photoelectrons detected per pixel, n_(D) is the number ofdark current electrons per pixel, and n_(e) is the number of read noiseelectrons per pixel. The measured signal intensities and solarbackground was re-scaled to smaller aperture diameters (5 cm to 10 cm)and an exposure time of 5 milliseconds. The calculated values of the SNRfor three aperture diameters (D=10 cm, 7.5 cm, and 5 cm) and two cameras(with readout noise of 150 photoelectrons/pixel and 13photoelectrons/pixel) at sea level and 20,000 ft altitude are shown inFIGS. 14, 15, and 16 versus H-band star magnitude. For the 10 cmaperture diameter telescope, the exposure time is 5 msec, the pixelfield of view is 16 microradians, and the angular distance from the sunis 80°. For the 7.5 cm aperture diameter telescope, the exposure time is5 msec, the pixel field of view is 21 microradians, and angular distancefrom the sun is 80°. For the 5 cm diameter telescope, the exposure timeis 5 msec, the pixel field of view is 27 microradians, and angulardistance from the sun is 80°.

Using a commercial infrared camera from CEDIP Infrared Systems (JADESWIR HgCdTe focal plane array, 320×2450 pixels, 1.2 million electronsfull well capacity, 150 electrons read out noise, thermoelectric cooler)and a 10 cm telescope at sea level, the sensor can detect (with SNRgreater than 10) stars equal or brighter than 2.3 magnitude during theday. At night, much dimmer stars are visible. Using a 5 cm aperturetelescope, the daytime star detection limit at sea level is the firstH-band star magnitude. According to Table 1, on average there are threefirst H-band magnitude stars in the 15°×15° search window. Using a lownoise infrared camera (13 photoelectrons/pixel), the daytime stardetection limit reduces to the second H-band magnitude. This shows thata small aperture (D=5 cm) optical GPS unit can operate at sea levelduring both daytime and night time.

First Compact Embodiment

The first compact embodiment of the present invention uses a singletelescope equipped with a two-axis high precision rotary stage to pointthe field of view at a selected bright infrared star within a searchwindow. It also includes a temporary mirror which turns the telescopefield of view by 90°. The latter is necessary because a geographicalposition determination requires that the star measurements be separatedby an angular distance of between 90° and 120° in azimuth.

This design concept is shown in FIG. 17. When a temporary mirror 50 isinserted into the optical train, the field of view is deflected by 90°.A small telescope 52, nominally 5 cm aperture, points toward a mirror 54mounted onto a precision stage 56 that has a 7.5° rotation capability,for example, the ARA125 from Aerotech, Inc. This rotary stage has anabsolute accuracy of 1 arc-second. The telescope is fitted with aninfrared camera to view stars within a field of view of nominally0.44°×0.4°. The telescope uses a negative Barlow lens to increase theeffective focal length while keeping the total length as short as 15 cm.The field of view of the telescope is changed in one direction by 15° byrotating the stage by 7.5°. The stage and mirror are connected to asecond precision rotary stage mounted at a right angle, allowing thetelescope to point directly to any particular stellar field within a 15°field in both directions.

After the stage-mounted mirror is the temporary mirror 50 mounted at afixed angle. This mirror can be quickly inserted into the optical path.The field of view of the telescope is then deflected by a fixed amount(nominally 90°), but is still able to point over a 15° field of view.The temporary mirror is inserted into a precise location to maintain theangular precision. This allows a single telescope and camera tosequentially view two fields of view. Alternatively, this temporarymirror may have two or more fixed orientations, including an angle 90°orthogonal to that shown. This provides even more sky coverage, which isespecially useful in the case of partly cloudy skies or when the solarangle is not far enough from the original directions. An advantage ofthis approach is that it has few components (a single telescope and onetwo-axis rotating stage). A possible disadvantage is that sequential(time separated) star measurements may be affected by the platformmotion and vibration more than the simultaneous measurements.

Second Compact Embodiment

The second compact embodiment of the present invention uses twoidentical telescopes mounted to a dual-axis precision rotary stage. Inthe design shown in FIG. 18, the two telescopes 60 and 62 are pointed at90° with respect to each other, sharing a common focal plane. Small foldmirrors near the focal plane, not shown, are used to combine the fieldsof view onto one infrared camera. An alternate design would add a 45°fold mirror at the output of one of the telescopes, so that the twotelescope bodies are essentially parallel, and the different fields ofview are provided by the 45° fold mirror. A single infrared camera isused for both telescopes. Two approaches for combining two images in asingle focal plane array have been described above and shown in FIG. 5B.This design also includes two temporary fold mirrors that can beinserted at a precise angle. These temporary mirrors increase the skycoverage for the cases where the solar angle is too close to theoriginal direction, or in the case of partly cloudy skies, where thereis an advantage to pointing toward holes in the clouds.

Benefits

The key benefits of the proposed strap-down stellar imaging system withlimited degree of freedom are:

-   -   compact sensor design (5 cm aperture, 20 cm×15 cm×12 cm        dimensions). Such a sensor with small dimensions is well suited        for navigating flying platforms such as unmanned arial vehicles,        aircrafts, and missiles,    -   light weight,    -   high probability of detecting stars because the telescope is        pointed at selected stars,    -   high update rate of the celestial fix (a few to several tens of        seconds), and    -   increased accuracy and reliability of the estimates for a        celestial fix.    -   This last advantage is due to two factors: a) increased signal        to noise ratio of the stellar imagery data and b) elimination of        the need for star identification using star pattern recognition        (there is no need to identify a star because a telescope will be        pointed at the selected star using a two-axis rotating stage).

Compact Sensor Operation

The sensor operates as follows. First, by using an infrared starcatalog, first H-band magnitude stars are selected within two searchwindows separated at 90° in azimuth. Second, using the local verticalmeasured with an on-board inclinometer as a reference (for example, theLCF 2000 series from Jewel Instruments), the sensor software generatescontrol commands for the two-axis rotary stage and points the telescopesat the selected stars. (Since the instantaneous field of view of thetelescope is 153 arc-seconds×192 arc-seconds, at this stage the starposition is known within about the same accuracy, i.e. 153arc-seconds×192 arc-seconds). Third, the sensor acquires the star imagesand processes the imagery data. Fourth, the sensor software calculatesthe image centroid with sub-arc-second accuracy. Then, the on-boardsoftware calculates the star height with respect to local horizon and aline of position (LOP). At the same time, the second telescope acquiresstar imagery within the second 15°×15° search window separated from thefirst search window at about 90° in azimuth, processes the imagery data,and calculates the second line of position. The intersection of the twolines of position defines a celestial fix (latitude and longitude).

The accuracy of the centroid measurements with an infrared sensor, aswell as the accuracy of the longitude and latitude calculations usingsite reduction software is a fraction of an arc-second. Thus, theaccuracy of the proposed stellar tracker is limited by the absoluteaccuracy of the rotary stage, which is one arc-second. Applicants expectthat an update rate for a celestial fix will be on the order of a fewseconds

In order to convert the measured angular positions of the stars into ageo position, accurate measurements of the local vertical are required.Applicants will use a commercial Jewel LCF-196 inclinometer. This sensoris designed for applications with high level of shock and vibration. Itsresolution is 3 micro-radian and the bandwidth is 30 Hz. With a lowerband width of 1 Hz, an accuracy of about 1 micro radian is estimated.

Advanced Sensor Design with an “All Sky” Camera

The first preferred embodiment shown in FIG. 4A includes threetelescopes mounted to have their optical axes separated at 120 degrees.The use of three telescopes allows for both increased position accuracydue to the ability to triangulate the measurements and redundancy incase one of the telescope optical axes is close to the sun. A similarapproach may be used in a compact unit. Two telescopes (fourfield-of-views using temporary mirrors) will share a single infraredfocal plane array. Two techniques may be used to combine the light fromeach of the two telescopes onto a single focal plane array. A preferredtechnique uses a small turning mirror and two-sided pyramid mirror tocombine the light from three telescopes. Another aperture combiningtechnique involves the use of a fiber image conduit.

Avoiding the Sun

In order to increase the operational utility of the above compactsystems, a visible-band “all sky” CCD camera may be included. Thisvisible camera would optimize the sensor performance. It would image thesky over an area of 180°×180° to determine the approximate sun positionwith respect to the optical axes of the one or more infrared telescopesas well as positions of clouds. Then, the sensor software will selectthose telescopes (one, two or three) whose optical axes are separated atangular distances from the sun greater than 80° and have a clear line ofsight. Thus, an “all sky” camera will allow users to minimize theeffects of clouds on the star imaging system. An Optical GPS unit willbe integrated with the on-board inertial navigation unit. As in theearlier described embodiments, if bad weather separates star sightings,the inertial navigation unit will carry the stellar fix forward untilnew star observations can be obtained.

Alternatives, Modifications and Variations

Marine Environment

A marine environment provides the challenge of the sensor operatingautonomously over large variations in humidity and temperature, alongwith requiring additional protection from condensation and corrosion dueto fog and saline conditions. Some of the modifications that could berequired would be to change the lens housing or mechanical structurematerial to lower the coefficient of thermal expansion in order tomaintain the system focus while operating over an increased temperaturerange. Additionally, the sensor covering will be reviewed to provide forincreased weatherproofing protection for the optical system. The lensassembly is preferably designed so that the system can be nitrogenpurged which will prevent condensation on the internal surfaces of theoptics. Similarly, the entire sensor head could also be nitrogen purgedor a desiccant material placed internally to reduce condensation. Alarge mechanical shutter assembly is preferably placed on each of thethree lens apertures to provide protection of the optics during periodsof rain, ice, or snow, fog when the system would be prevented fromoperating due to poor atmospheric transmission.

To increase the reliability and maintainability of the unit whiledeployed at sea, several other designs should be considered. The wirecabling connection between the sensor head and electronics can beredesigned to use a single fiber optic cable. This could be an importantupgrade for the sensor head to improve reliability and ruggedness whilereducing the possible electromagnetic interference from externalshipboard hardware such as radars. To improve the maintainability of thesensor, an increased set of built-in diagnostic capabilities could beimplemented for a deployed system. This would also include an autonomouscalibration diagnostic that can be run during favorable atmosphericconditions (clear night time) when the probability of observing severalstars in each aperture is high. This diagnostic would recalibrate theline of sight of each of the apertures with respect to each other andthe inclinometer by knowing the ship location via GPS. In this way, thesystem could autonomously calibrate out small thermal and/or mechanicaldrifts during periods of opportunity to increase the system reliability,maintainability and accuracy. The accuracy of the local horizonmeasurements using the inclinometer will require review. Specifically,the update rate requirement along with the suppression of angularacceleration effects should be reviewed. The addition of angular ratesensors may be required to permit removal of platform motion effects inmultiple frame averages.

Aircraft Issues

Peculiarities of the present invention for the aircraft include:

-   -   a) Effect of atmospheric obscurants including clouds is reduced        (50% probability of clear line of site at sea level, and 90%        probability at 30,000 ft). The use of multiple measurement        channels increases the probability of clear line of sight;    -   b) Daytime sky background is reduced by a factor of 10 for every        20,000 ft;    -   c) Simultaneous measurements with four optical channels may be        preferred to reduce the effect of aircraft vibration and motion;    -   d) Short exposure time (1 msec or lower) may be required to        prevent star blurring due to aircraft vibration; and    -   e) A multiple-frame averaging technique should be used to reduce        noise and increase the signal to noise ratio in the imagery        data.

For the aircraft application, it is likely important to reduce the sizeand weight of the unit while also having an increased vibrationaloperating specification for the sensor. Due to reduced sky background ataltitude, the sensor apertures could be designed for a smaller diameterwith a shorter focal length to maintain the same f-number. Similarly,the mechanical structure could be designed with composite materials toincrease stiffness and reduce susceptibility in a harsh vibrationalenvironment, lower sensor head weight, and reduce the system thermalsusceptibility.

Although the present invention has been described above in terms ofspecific preferred embodiments persons skilled in this art willrecognize that many changes and variations are possible withoutdeviation from the basic invention. For example, platform position canbe determined with only two telescopes. With three telescopes at leasttwo will always be pointed more than 30 degrees away from the sun. Ifonly two telescopes are used, preferably they would be mounted with a 90degree azimuthal separation at an elevation of 45 degrees to thehorizon. There could also be situations where four telescopes would bepreferred. Many infrared sensors other than the ones specificallyreferred to are available for operation in the transmission windowsshown as 4 and 6 in FIG. 2A. The systems could have applications otherthan ship or airplane navigation. Various additional components could beadded to provide additional automation to the system and to displayposition information. Star catalogs may include celestial objects otherthan stars such as planets and asteroids. Otherwise, if one of theseobjects shows up in an image, it could confuse the system. The CCDcamera discussed above for looking for cloudless regions of the sky (oranother visible light camera) can be very useful for both the strap-downembodiments as well as the embodiments with the pointing telescopes. Totake advantage of clear regions of the sky for the strap-downembodiments three temporary fold mirrors such as the ones described forthe pointing telescopes may be provided for temporary insertion in eachof the three strap-down telescopes to change each field of view by fixedangles. The computer processor will be programmed to cause the insertionof the temporary fold mirrors if the CCD camera daatta shown that theinsertion will provide a clearer line of sight to infrared stars.Accordingly, the scope of the invention should be determined by theappended claims and their legal equivalents.

1. An automatic celestial navigation system for navigating both nightand day by observation of K-band or H-band infrared light from multiplestars, said system comprising: A) at least one telescope on a movableplatform for viewing at least two substantially different portions ofsky with each of said at least one telescope defining a field of viewand comprising: 1) telescope optics for focusing H-band or K-band lightfrom stars in said field of view onto a focal plane, 2) a sensor locatedat said focal plane for detecting H-band or K-band light, B) an accuratetiming device, C) an accurate inclination sensor, and D) a computerprocessor having access to infrared star chart information, programmedto compare image data from said infrared sensors, time information fromsaid timing device, inclination information from said inclination sensorand said star chart information to determine positions of said platform.2. The system as in claim 1 wherein said at least one telescope is twotelescopes.
 3. The system as in claim 1 wherein said system is a compactsystem suitable for use on a missile, an unmanned aerial vehicle orother aircraft and having a volume of substantially less than one cubicmeter.
 4. The system as in claim 3 wherein said system further comprisesa rotary means for pointing the field of view of said at least onetelescope with an accuracy of less than 3 arc-seconds.
 5. The system asin claim 3 wherein said system further comprises a rotary means forpointing the field of view of said at least one telescope with anaccuracy of about 1 arc-second or better.
 6. The system as in claim 3wherein said system further comprises two rotary stages for pointing thefield of view of said at least one telescope.
 7. The system as in claim6 wherein said at least one telescope is two telescopes.
 8. The systemas in claim 6 wherein said at least one telescope is three telescopes.9. The system as in claim 1 wherein said at least one telescope is threetelescopes and each of said three telescopes are rigidly mounted withrespect to the platform and each other.
 10. The system as in claim 1 andfurther comprising temperature and pressure sensors wherein saidprocessor is also programmed to utilize information from saidtemperature and pressure sensors to correct for variations caused bychanging temperature and pressure.
 11. The system as in claim 9 whereineach of said three telescopes are pointed in azimuthal directions spacedat about 120 degrees relative to each other and at about 45 degreesrelative to horizontal.
 12. The system as in claim 1 wherein said systemis integrated with an inertial guidance system.
 13. The system as inclaim 9 wherein said platform is a ship or a portion of a ship.
 14. Thesystem as in claim 3 wherein said platform is a ship or a portion of anairplane.
 15. The system as in claim 3 wherein said platform is a shipor a portion of a missile.
 16. The system as in claim 3 wherein saidplatform is a ship or a portion of an unmanned aerial vehicle.
 17. Thesystem as in claim 1 wherein each of said sensors have full wellcapacities in excess of 5 million electrons.
 18. The system as in claim1 wherein each of said sensors have a frame readout period of less than30 milliseconds.
 19. The system as in claim 9 wherein each of saidtelescopes have a pixel size of about 6 arc-seconds.
 20. The system asin claim 3 wherein each of said telescopes have a pixel size of about 16to 27 microradians.
 21. The system as in claim 1 wherein said processoris programmed to detect stars in the presence of strong background noiseand to reduce noise, by utilizing multiple individual frames that arealigned to a star image in a single frame and summed up.
 22. The systemas in claim 1 wherein sizes of search windows are determined by the rootmean square error of measurements of local verticals with aninclinometer.
 23. The system as in claim 1 wherein stars are identifiedusing angular distances between stars, star relative brightness andtriangle patterns.
 24. The system as in claim 9 wherein each of saidthree telescopes have an instantaneous field of view of about 0.4×0.5degrees.
 25. The system as in claim 9 wherein said computer processor isprogrammed to determine position utilizing the following steps: A) toreduces the fixed pattern camera noise, considering the data set thatconsists of 20 frames and first, using 20 data frames that precede thefirst frame in the data set, the median data frame is calculated todetermine a background threshold for each pixel, then, the median frameof pixels is subtracted from each frame in the data set, pixel by pixel;B) next, to reduce noise, five sequential data frames in the data setare blindly summed up to spreads star illumination over a few pixels; C)then a “super pixeled” image is created by down sampling the imagegenerated in step 2 at the rate of 1:4 (i.e., four adjacent pixels aresummed across the pixel array); D) determine the brightest super pixelin the frame where the star can be located and create a small (9×9regular pixel size) window about the brightest super-pixel location (81pixels with the brightest 4×4 in the middle); E) to increase thecentroid accuracy, up sample the image within the window at the rate10:1 using cubic spline fit algorithm image at the noise ceiling (i.e.,the computer produces a digital array of 90×90 [8100] virtual pixels andfits them with the cubic spline fit algorithm into a Gaussian-likeshape); F) calculate the intensity weighted centroid to determine anexpected star location in the first data frame; G) repeat steps B)-F)for each subsequent data frames in the data set; H) once an expectedstar location in all subsequent data frames are determined, shift all 20frames to the star position in the first frame, and sum up all frames tocreate a final image for the entire frame, based on the shifts obtainedfrom the 9×9 pixel centroiding window, the extent of the shift is beingbased on the location of the centroid; I) create a “super pixel”representation of the shifted and added frame obtained from step 8 bydown sampling at the rate of 1:4 (i.e., four adjacent pixels are summedacross the pixel array), determine brightest super pixel and create asmall (9×9 regular pixel size) window about that location; J) up samplethe image within the small 9×9 pixel window with a ratio of 10:1 byusing cubic spline fit algorithm, remove background by chopping at noiseceiling, calculate intensity weighted centroid position as well as totalintensity in the image, make an estimation of the rms noise by takingthe standard deviation σ of all pixels [other than pixels illuminated bybright stellar objects] in the entire image frame, remove the datawithin the small window in order to search for the next dimmest star,repeat steps I and J until all potential stellar objects within theframe are found; K) for each potential star location, the pixel SNR iscalculated:${{SNR} = \frac{I_{S} - \left\langle I \right\rangle}{\sigma}},$  whereI_(s) is the total signal intensity divided by number of pixels in theimage, <I> is the mean intensity in the image, and σ is the rms noiseand if the SNR≧10, then the star is detected and the star coordinatesare determined by intensity weighted centroid calculated in step J) butif SNR<10, then this potential star location is rejected and treated asa noise; and L) star coordinates along with the star intensity are usedfurther by automated star pattern recognition algorithm and coordinatesof the brightest star in the field of view are used in calculations oflatitude, longitude and absolute azimuth.
 26. The system as in claim 3wherein the system is integrated with an inertial navigation systemusing a Kalman filter.
 27. The system as in claim 1 wherein saidcomputer processor is programmed to determine a absolute azimuthreference utilizing at least one star position.
 28. The system as inclaim 27 wherein said absolute azimuth reference is utilized todetermine attitude of said platform.
 29. The system as in claim 28wherein the platform is an artillery gun.
 30. The system as in claim 1and further comprising a visible light camera especially for use onpartly cloudy days for determing directions to cloudless regions of thesky.
 31. The system as in claim 30 wherein the visible light camera is aCCD camera.
 32. The system as in claim 9 wherein said three rigidlymounted telescopes each comprise a temporary fold mirror for changingthe fields of view of each telescope by fixed angles.